The invention relates to an electrically heating hair curling wand, particularly to the electric resistor means adapted to heat the barrel of the wand to a required temperature. It relates more particularly to resistor means adapted to heat the central portion of the barrel to a uniform temperature, and to maintain this temperature during the entire curling operation.
A hair curling wand consists essentially of a handle, a tubular barrel attached to one end of the handle, usually concentric therewith, and an electric heating element inserted into the barrel and energized before and during the use of the curling wand. the appliance is furthermore provided with means of different configuration for firmly gripping the hair strand wound around the barrel; it may consist of a srping-loaded clip adapted to forcefully enbrace a portion of the barrel circumference, of rows of teeth or combs attached to, or integral with, the barrel, or of bristles attached to the barrel thus forming a cylindrical brush.
The appliance serves to curl a strand of hair closely wound around the heated barrel and held in position by one of the hair gripping means; the barrel heat, assisted by various cosmetic liquids applied to the hair, keeps the strand in its curled state after its release from the wand. In this connection it should be noted that the barrel is usually made longer than the width of the hair strand wound around it, and that, for this reason, only the central portion of the barrel is heated to the highest allowable temperature, while the end attached to the handle as well as the far end remain relatively cool. The heating elements used with the conventional curling wands are of two kinds:--1. a so-called rope-heater inserted into the central portion of the barrel and extending over about 2/3 of the barrel length. This kind suffers from the following drawbacks:--contact between the barrel inside surface and the rope heater is only along a series of points, instead over the entire resistor surface; this results in inadequate heat transfer and, therefore, in slower heat-up time. It also requires a barrel of relatively great wall thickness, in order to equalize the temperature over the entire area. Another drawback of the ropeheater type is that a maximum temperature is reached in the central portion of the barrel and decreasing from there to both ends; and 2. A "positive-temperature-coefficient" element (PTC) which consists of a semi-conductor embedded in a cylindrical body of an inslating material of a diameter corresponding to the inner diameter of the barrel and occupying about 50% of the barrel length. While this heating element presents considerable advantages compared with the rope-heater, particularly in regard to the danger of overheating and in respect of contact along one or more lines between heater surface and barrel interior instead of in several points only, its high cost is a deterrent against its use in low-cost curling irons. And here again, as with the rope-heater, a relatively thick-walled barrel tube becomes necessary in order to obtain uniform distribition of temperature, since due to the nature of the PTC-element a maximum temperature is reached in the central portion of the element--and of the barrel--from where it gradually decreases to both ends of the barrel.
The present invention has therefore, as its main object to provide a heating element adapted to be inserted into the barrel so that intimate contact with the barrel inside is ensured over the entire surface of the heating element, with the result that a thin-walled barrel tube can be utilized.
It is another object to provide a heating element of a resistor material having a high positive thermal coefficient of resistance, so as to dispense with a thermostatic switch and at the same time prevent overheating of the barrel.
And it is a final object of the invention to provide a heating element which will emit a larger amount of thermal energy in its end portions than in its central portion, the energy distribution being so designed that the barrel portion containing the heating element should be heated to substantially uniform temperature.